Screw rotor machines employable, both for compression or expansion of an elastic working fluid, have used asymmetric rotor profiles for improved efficiency of the compression or expansion process. The development of asymmetric screw rotor profiles is exemplified by U.S. Pat. Nos. 3,423,017, 4,140,445, and 4,435,139, assigned to Svenska Rotor Maskiner; U.S. Pat. Nos. 4,053,263 and 4,109,362 issued to the present applicant, and assigned to the Joy Manufacturing Company; and U.S. Pat. Nos. 4,401,420 and 4,406,602 assigned to the Hitachi Corporation.
Screw rotor machines, whether functioning as compressors or expanders, are formed normally of a cast or machined casing or housing bearing two parallel, laterally intersecting cylindrical bores opening at respective ends to high pressure and low pressure ports. Within the bores, there are mounted for rotation, interengaging helical screw rotors of the male and female type provided with helical lobes or lands and intervening grooves having wrap angles normally less than 300 degrees. Typically, the male rotor is a rotor in which each lobe and groove has at least its major portion located outside the pitch circule of the rotor and has two generally convex flanks located outside the pitch circle, while the female rotor comprises a rotor in which each lobe and groove has at least its major portion located inside the pitch circle of the rotor and has two generally concave flanks located inside the pitch circle of the rotor.
Particularly in the smaller size compressors and expanders, it is preferred that the female rotor physically drive the male rotor. Typically, the female, driving rotor may consist of six lobes, while the male, driven rotor may constitute four lobes and thus be rotated at one-half greater speed. In such female drive arrangement, the majority of lobe action occurs in the arc of approach or behind the line of centers. This is a particularly destructive type of lobe action, because the direction of sliding of one lobe surface on the other is towards the pitch diameters and results in a spreading force which opposes rotation. With respect to SRM U.S. Pat. Nos. 3,423,017 and 4,140,445, Joy U.S. Pat. Nos. 4,053,263 and 4,109,326 and Hitachi U.S. Pat. Nos. 4,401,420 and 4,406,602, this condition exists.
Regarding the more recently issued U.S. Pat. No. 4,435,139 of SRM, while the rotor profiles are exemplified as favoring a female drive situation and a minimization in the destructive type of lobe action, that patent, in achieving a smooth profile surface, utilizes a relatively small radius forming a portion of the lobe surface considerably below the pitch circle which results in a blow hole which is considerably larger than that which is present in its earlier screw rotor profiles, as exemplified by U.S. Pat. Nos. 3,423,017 and 4,140,445.
In both Hitachi U.S. Pat. Nos. 4,401,420 and 4,406,602, relatively small blow holes are achieved by the use of very small tip radii and very small or in some cases negative female addendums. This results in a virtually impossible situation if the female rotor is functioning to drive the male rotor. In SRM U.S. Pat. Nos. 3,423,017 and 4,140,445, as well as my earlier U.S. Pat. Nos. 4,053,263 and 4,109,362, a sharp line occurs on the female rotor where the point generated dedendum portion intersects the profile near the pitch circle. This also equates to the small radii characterized by the profiles of the Hitachi U.S. Pat. Nos. 4,401,420 and 4,406,602, and the result is a highly destructive wear problem for the interconnected rotors.
In an attempt to minimize wear as a result of such interference, localized hardening is necessary at the pitch surfaces of most female drive rotors to insure sufficient life to the helical screw rotors to justify their cost.
It is, therefore, a primary object of the present invention to provide an improved helical screw rotary machine having rotor profiles which improve meshing conditions, minimize leakage paths in the compressor or expander, improve cutting conditions in the manufacture of the rotors, and wherein the requirement for laser or other localized hardening of the lobes in the vicinity of the pitch surfaces is substantially eliminated.